In the filtration of gasses or liquids the pore size of the filter media used is typically proportional to the size of the particulates to be removed. Since the differential pressure across the filter media is inversely proportional to the pore size, cycle times increase inversely with the pore size of the filter media and the energy expended also increases as the pore size of the filter media is decreased. As a consequence, shorter cycle times and subsequently increased energy use have been required in the filtration of gasses contaminated with very fine particulates.
The filtration of hot gasses in which very fine particulates are entrained has been carried out with the use of porous ceramic filter tubes having a relatively coarse supporting matrix portion over which there is a relatively thin filter layer of a very fine porous ceramic. Even with such filters the filter cake which builds up on the surface of the fine filter layer decreases the operating cycle by increasing the pressure drop across the filter. If there is a relatively great amount of particulate in the gas the resultant filter cakes can damage the filter by filling the space between adjacent filter tubes. Moreover, the very fine particles penetrate the interstices in the filter layer and irreversibly plug or foul the filter element.
In order to alleviate the problems inherent with the filtration of the fine particulates and thus to increase the efficiency of the system, one or more centrifugal separators or cyclones have been connected in the system upstream of the filter to remove the majority of the particulate contaminant mass from the gas stream prior to the final filtration or polishing operation.
We have found that the particulates in the gas stream exiting the cyclones sometimes have an abnormally small particle size distribution, i.e., they have an unusually high fraction of very fine solids in the range of 0.15 to 1.9 microns. If a filter media having a pore size small enough to remove these very fine particulates is used, the pressure drop across the filter becomes excessive for the efficient operation of the system. In the alternative, if a filter media having a mean pore size which is suitable for use with a normal particle size distribution is used, wherefore the pressure drop is not excessive, these very fine particles pass into the interstices of the filter media where they become permanently lodged so that in a relatively short time the filter media must be replaced. We have found a solution to this problem.